WO1991010899A1

WO1991010899A1 - Process and device for determining amplitude and phase values according to a reflectometric measurement process

Info

Publication number: WO1991010899A1
Application number: PCT/AT1990/000129
Authority: WO
Inventors: Werner Pritzl
Original assignee: Werner Pritzl
Priority date: 1989-12-29
Filing date: 1990-12-28
Publication date: 1991-07-25
Also published as: ATA297489A; AT397584B; NO922547D0; ATE118613T1; EP0508992A1; FI923008A0; NO922547L; EP0508992B1; FI923008A; DE59008493D1

Abstract

An arrangement determines amplitude and phase values of two signals of differing polarisation received by two separate receiving antennas, in particular signals in the micro-wave range, of a reflectometric measurement device. In order to obtain a simple design, the receiving antennas (5, 6) are connected with the inputs of a power divider (9, 19, 11, 12) and power combiner (13, 14), the four outputs of which are connected with detectors (20, 21, 22, 23). Two connectors directly receive the signals of an associated receiving antenna (5, 6) and the two other detectors receive a mix of such signals, one of the latter detectors (21, 22) receiving a mix of such signals the phase angle of one of which is rotated. A process for calibrating this arrangement is also described.

Description

Device and method for determining amplitude and phase sizes in reflectometric measurement methods

The invention relates to a device for determining the amplitude and phase size of two differently polarized signals picked up by two separate receiving antennas, in particular signals in the microwave range, a reflective measuring device, and a method for calibrating this device.

Refractometry is a known measuring method for determining the refractive index or dielectric constant of surfaces or, in an extension of the method for determining refractive index or dielectric constant and layer thickness of a layer covering a surface. In this method, signals formed by electromagnetic waves, usually at an acute angle, are radiated onto the surface to be examined and the reflected wave or the reflected signals are recorded by means of a suitable receiver arrangement and the amounts of the p- and s-polarized portions of the Signals and the phase difference between the p- and the s-polarized components determined.

As is known from US Pat. No. 4,052,666, other unknowns occur when determining the quantities sought, such as the real and imaginary parts of the refractive index or dielectric constant and the thickness of the layer covering the surface. These are quantities describing the reflection behavior of the surface, e.g. in the case that there is a homogeneous dielectric under the surface, around the real and imaginary part of the refractive index or dielectric constant of the material. In the general case, five unknown sizes are involved in the measurement. Even if the received signal is polarized in two mutually orthogonal directions and analyzed for magnitude and phase difference, there remain at least two quantities that cannot be determined from the available data, so that the entire system of equations remains indeterminate, or certain assumptions have to be made, which means that corresponding ones Errors can result.

However, if it is possible to determine the reflection behavior of the surface solely on the basis of magnitude and phase difference, then the complex refractive index or dielectric can number of surfaces can be determined separately. With the knowledge of these variables, it is possible in a subsequent measurement, in the presence of a layer covering the surface, to also clearly determine the remaining unknowns.

However, if the surface delimits an inhomogeneous material, the reflection behavior of the surface can no longer be characterized by the complex refractive index or dielectric constant. In this case, the reflection behavior of an electromagnetic wave can only be described by two complex reflection factors, each for the p- and s-polarized portion of the wave. This results in four unknowns, which consist of the three measurement sizes, etc. The amplitude of the p- and s-polarized components and the phase difference between these components cannot be determined directly. The same difficulty also arises when it is not possible to separately determine the reflection behavior of the surface.

If e.g. Microwave reflectometry for determining the nature and thickness of layers covering traffic areas, such as water, saline solutions, ice, snow, etc., as is proposed by PCT / AT 84/00041, the roadway can generally not be considered as a homogeneous medium to be viewed as. The difficulty described above therefore arises when determining the sizes sought, such as the real and imaginary parts of the refractive index or dielectric constant and the thickness of the layer covering the traffic area.

The aim of the invention is therefore to propose a device with which it is possible to determine a sufficient number of measurement sizes in order to be able to determine the refractive index or dielectric constant and the layer thickness using a suitable calibration and evaluation method.

This is achieved in accordance with the invention in that the receiving antennas are connected to the inputs of a circuit having a power divider and power combiner, the four outputs of which are connected to detectors, two of which directly apply the signals to an associated receiving antenna and the other two to a mixture of these signals are, wherein one of these latter detectors is subjected to a mixture of these signals, in which one of the two signals is rotated in phase.

Through these measures the different succeed Reflection of the p- and s-polarized portion of the electromagnetic wave forming the signal according to the following relationships

A = A _o • s in (ω t + φ) and

B = B _o • sin (ωt)

according to the amounts A _o and B _o and the phase difference φ.

Four different voltages can be tapped from the detectors, from which, as will be explained with reference to the description of the figures, taking into account calibration coefficients, the amounts of the reflection factors for the p- and s-polarized portion of the wave and the phase difference between the p- and the s -polarized portion of the wave, as will also be explained with reference to the description of the figures, can be determined.

According to a further feature of the invention, it can be provided that the power dividers connected on the input side to the receiving antennas are connected with their one outputs to one detector each and the second outputs of these power dividers are connected to the inputs of further power dividers, the one outputs of which each have an input assigned power combiner and each of the second outputs of the power divider is connected to that power combiner whose other input is connected to the other power divider, a phase shifter being arranged in at least one of these lines and the outputs of the power combiner being connected to detectors.

This results in a very simple construction of the device according to the invention.

It can further be provided that attenuators are connected upstream of the detectors, which are preferably formed by diodes.

These measures make it possible to limit the signal supplied to the detectors to values that lie in a specific section of the characteristic curve of the detector. This avoids errors due to deviations from an assumed characteristic. For example, in the case of detectors with a characteristic curve that is practically square in a certain area, the measures proposed can ensure that the signals applied to the detectors enter the range.

According to a further feature of the invention it can be provided that the phase shifter connected between the one power divider and a power combiner causes a phase shift of -90 °.

These measures result in particularly simple relationships with regard to the determination of the data describing the reflection.

Another object of the invention is to propose a method for calibrating the device according to the invention.

According to the invention, it is proposed that in a first step the receiving antennas and the transmitting antenna, which excite both polarization directions, are mutually aligned.

In order to take into account the shorter path of propagation between transmitting and receiving antennas when calibrating compared to measuring the reflection factors, the amplitudes A and B must be on

are normalized, where D is the distance between the antennas and H is the normal distance of the receiving antennas from the surface whose reflection is to be examined. This standardization ensures that the amplitudes Ao and Bo correspond directly to the amounts of the reflection factors in the later reflection measurement.

Now one of the inputs connected to an antenna in the operating state is terminated without reflection and thus one of the input signals, e.g. B, set to 0, in order to determine the distribution of the other signal, in this case A, between the four outputs by measuring the voltages at the four outputs with detector diodes.

In a further step, the distribution of the other signal between the four outputs can be determined by reflection-free termination of the other input and reconnecting the first input to the antenna.

In a last step of the calibration, in which the antennas are still aligned directly with one another, both inputs are again connected to the respective antennas bound. Since there is no reflection in this case, the phase shift is also φ = 0 and all phase shifts can be determined within the reception maintenance.

In a further development of this method, the procedure according to the invention can be such that the antennas (4), (5) and (6) are brought into their final alignment and the amounts for both the p- and the s-polarized portion of the wave of the s- and p-polarized parts of the wave and the phase difference between the s- and p-polarized parts of the waves reflected from the uncovered surface and then the amounts of the s- and p-polarized parts of the wave and the phase difference between the s- and p-polarized fractions of the waves reflected from a surface covered with a layer with a known refractive index or dielectric constant, preferably rainwater, the magnitude and phase of the reflection factors of the uncovered surface, eg a dry roadway. As a result, all fixed parameters are available, which serve subsequent ongoing measurements (operating status).

The invention furthermore relates to a measurement method carried out with the device according to the invention calibrated according to the above method, which according to the invention consists in the amounts of the s- and p-polarized portions of the wave and the phase difference between the s- and p-polarized Proportions measured with reference to the surface covered with a layer and from these measured values, taking into account the complex reflection factors of the uncovered surface, both the real and imaginary part of the refractive index or dielectric constant and the thickness of the layer can be determined. This is e.g. an ongoing measurement is possible in a particularly simple, in particular fully automated manner.

The invention will now be explained in more detail with reference to the drawing, which shows an embodiment of a device according to the invention.

The inputs 7 and 8 of the two power dividers 9 and 10 are connected to the two receiving antennas 5 and 6 which are polarized orthogonally to one another and which receive signals which are emitted by the transmitting antenna 4 and reflected on the surface to be examined and which are in the form of microwaves Reflection of the signals at the interface between the media 1 and 2 and also after penetration of the layer 2 on the surface of the medium 3 takes place. This occurs when the signals enter layer 2 Refraction. As a result, the signal reflected on the surface of the medium 2 and the signal reflected on the surface of the medium 3 arrive at the two receiving antennas 5 and 6 at different times.

The signals A and B present at the reception inputs 7 and 8 correspond to the following relationships:

A = A _o • sin (ω t + φ _OA + φ) and

B = B _o • cos (ωt + φ _OB )

The amplitudes A _o and B _o characterize the difference in reflection of the p- and s-polarized component of the electromagnetic wave, which was emitted by the transmitting antenna 4, while the angle φ is the phase difference between p- and s-polarized component indicates. The angles _OA and _OB indicate the respective phase shifts on the way of the signals from the receiving antennas 5 and 6 to the inputs 7 and 8 of the circuit having power dividers and power combiners.

This circuit has six power dividers, preferably 3 dB power dividers or power combiners 9 to 14, and a phase shifter 15.

The one outputs of the power dividers 9 and 10 are connected directly to the outputs 24 and 27. The second outputs of the power dividers 9, 10 are connected to further power dividers 11 and 12. The one outputs of the power dividers 11 and 12 are each connected to an input of a power combiner 13, 14. The second output of the power divider 11 is connected to the power combiner 14, the other input of which is connected to the power divider 12. The power divider 12 is connected to the power combiner 13 via a phase shifter 15, the other input of which is connected to the power divider 11.

Furthermore, four microwave detector diodes 20 to 27 are provided, which deliver rectified RF signals to the outputs 24 to 27, but attenuators 16 to 19 are connected upstream of the detector diodes 20 to 27.

The attenuators are dimensioned so that the detector diodes 20 to 23 can be operated in the square area of their characteristic. At the outputs 24 and 27, quadratic characteristics of the detector diodes 20 to 23 are preceded sets, voltages X, T occur which are proportional to A _o ² and B _o ² , whereby

X = k _AX • A _o ² and

T = k _BT • B _o ² corresponds. K _AX and k _BT mean calibration coefficients.

The output voltages at the outputs Y and Z are again provided that the characteristic curves of the detector diodes 21 and 22 are square,

Y = k _AY ² A _o ² + k _BY ² B _o ² + 2k _AY k _BY A _o B _o COS (φ + (φ _AO - φ _BO ) +

(φ _AY -φ _BY )) and

Z = k _AZ ² A _o ² + k _BZ ² B _o ² + 2 k _AZ k _BZ A _o B _{o cos} (φ + (φ _AO -φ _BO ) +

(φ _AZ - φ _BZ ))

wherein the phase difference between the input 7 and the detector diodes 21 and 22 with t φ _AY and φ _AZ and the phase difference between the input 8 and the detector diodes 21 and 22 are denoted by φ _BY and φ _BZ .

Now take into account that apart from manufacturing tolerances φ _AY , φ _AZ and φ _{BZ are the} same size and the phase shifter 15 causes a phase shift of -90 ° and thus gives φ _BY φ _BZ - 90 °

(φ _AZ - φ _BZ ) - (φ _RY -φ _BY ) = -90 ° + ε where ε describes a small angle that includes all of the manufacturing tolerances mentioned for the power divider 9 to 14, the attenuators 16 to 19 and the phase shifter 15.

If you continue with the designation

Δφ = (φ _AO -φ _BO ) + (φ _AY -φ _BY )

a, the simpler relationships result for the output voltages Y and Z at the outputs 25 and 26

Y = k _AY ² A _o ² + k _BY ² B _o ² + 2k _AY k _BY A _o B _o cos (φ + Δφ) and

Z = k _AZ ² A _o ² + k _BZ ² B _o ² + 2k _AZ k _BZ A _o B _o sin (φ + Δφ + ε) it is again assumed that the detector diodes 21 and 22 are operated in the square region of the characteristic.

In the receiver network shown in the drawing, 24 to 27 electrical voltages X, Y, Z and T are available at its outputs, with the aid of which the amplitudes A _o and B _o as well as the phase difference φ can be clearly determined, provided that the calibration coefficient cient k _AX , k _AY , k _BY , k _AZ , k _BZ , k _BT , Δφ and ε are known.

With the aid of a method according to the invention for calibrating the device shown in the drawing and determining the three measurement sizes, etc. the amount of the p- and s-polarized component and phase difference as well as the phase difference between p- and s-polarized component and in a further step the unambiguous determination of the quantities sought, such as a complex refractive index or dielectric constant and the thickness of the layer is possible .

To carry out the calibration method according to the invention, the transmitting antenna 4 and the two receiving antennas 5 and 6 are aligned directly with one another, so that the emitted signals in the form of electromagnetic waves strike the receiving antennas 5 and 6 directly. The following values are assigned to the amplitudes A _o and B _o during calibration:

In a later reflection measurement A _o and B _o thus directly correspond to the amounts of the reflection factors. The distance between the receiving antennas is denoted by D, the normal distance of the antennas from the surface of the medium 2 by H.

For example, if D = 10m and H = 4m, the amplitudes must be assigned the value 1.2806.

First, the receiving antenna 5 is connected to the input 7 of the circuit having power dividers and power converters, and the second input 8 is terminated without reflection. Since the signal B = 0, the calibration coefficients k _AX , k _AY and k _AZ result from this measurement: X = k _AX • A _o ² Y = k _AY ² • A _o ²

Z = k _AZ ² • A _o ²

T = 0

If the voltages are, for example, X = 1.05 mV, Y = 0.24 mV and Z = 0.27 mV, the result for calibration coefficients

k _AX = 0.6402 mV, k _AY = 0.3825 and k _AZ = 0.4058

Conversely, if input 7 is closed without reflection and the receiving antenna 6 is connected to input 8, the calibration coefficients k _BY , k _BZ and k _BT can be determined analogously.

X = 0

Y = k _BY ² B _o 2

Z = k _BZ 2 B _o 2

T = k _BT B _o 2

For example, if the voltages are Y = 0.25 mV, Z = 0.26 mV and T = 0.95 mV, the calibration coefficients result

k _BY = 0.3904 k _BZ = 0.3982 and k _BT = 0.5793

mV.

In a third step, the receiving antenna 5 is connected to the input 7 of the circuit having power dividers and power converters, and the receiving antenna 6 is connected to the input 8. The remaining unknown calibration coefficients Δφ and ε result from this measurement, since in this case φ = 0: Y = k _AY ² A _o ² + k _BY B _o ² + 2 k _AY k _BY A _o Bo cos (Aφ),

Y - k _AY ² A _o ² ~ k _BY ² B _o ²

Ao = Bo cos Δφ = ------------------------------------

2k _AY k _BY A _o B _o

Z = k _AZ ² A _o ^{2 +} k _BZ ² B _o ² + 2k _AZ k _BZ A _o B _o sin (Δφ + ε),

Zk _AZ ² Ao ² -k _BZ ² B _o ²

Ao = B _o sin (Δφ + ε) = -------------------------------

2k _AZ k _BZ A _o B _o and from this Δφ and ε can be determined.

For example, if the voltages are Y = 0.18 mV

and Z = 0.92 mV, we get cos (Δφ) = - 0.6327 and sin

(Δφ + ε) = 0.7356. From cos (Δφ) = - 0.6327 it now follows that

= 129.25 ° or -129.25 °. But since sin (Δφ + ε) »0,

only Δφ = 129.25 ° is possible as a solution. Analogously it follows

sin (Aφ + ε) = 0.7356 and cos (Δφ) «0 that Δφ + ε = 132.64 °

and thus ε = 132.64 ° -Δφ = 3.39 °.

All calibration coefficients of the device according to the invention are now determined by means of this calibration rule and, after the antennas 4 to 6 are aligned with the surface to be measured, e.g. a roadway, the measurement of the amplitude of the p- and s-polarized portion of the transmitted signal and the phase difference between p- and s-polarized portion possible.

For the calculation of the sizes A _o , B _o and φ from the

The measured voltages are carried out as follows:

First the amplitudes A _o and B _{o are determined} from the output voltages X and T:

X = k _AX A _o ²

T = k _BT B _o ²

So that from the output voltages Y and Z Y - k _AY ² A _o ² - k _BY B _o ²

COS (φ + Δφ) = ------------------------------------------- ------

2k _AY k _BY A _o B _o

Zk _AZ ² A _o ² - k _BZ ² B _o ²

sin (φ + Δφ ε) = ------------------------------------------ -------------

2k _AZ k _BZ A _o B _o

be calculated. With the relationship

sin (φ + Δφ + ε) = sin (φ + Δφ) • cos ε + cos (φ + Δφ) • sin ε sin (φ + Δφ + ε) - cos (φ + Δφ) • sin ε sin (φ + Δφ) = ----------------------------------------------- ----------------- you get

s in (φ + Δφ) 1 s in (φ + Δ φ + ε)

tan (φ + Δ φ) = ------------------------- = --------- ------- ------------------------ - tan ε cos (φ + Δφ) cos ε cos (φ + Δφ)

From tan (φ + Δφ) and cos (φ + Δφ) φ + Δφ and thus φ can be clearly determined.

For example, if the output voltages are X = 0.0250 mV, Y = 0.0696 mV, Z = 0.0634 mV and T = 0.1450 mV, A _o = 0.1976 and B _o = 0.5003. Furthermore, one obtains cos (φ + Δφ) = 0.8718 and sin (φ + Δφ + ε) = 0.5409 and thus tan (φ + Δφ) = 0.5623. Since cos (φ + Δφ)> 0 results from tan (φ + Δφ) = 0.6095 for φ + Δφ = 29.34 ° and for φ = 99.9 °.

Another goal is to determine the reflection factors of the uncovered surface of the medium 3, for example the dry road, by amount and phase, which allows the measurement of the amounts of the s and p polarization and the phase difference between the s and p polarization the surface covered with a layer of crushing or Determine the dielectric constant of the layer and the layer thickness.

As already explained, it is absolutely necessary to know the reflection properties of the surface in order to determine the sizes sought, such as the refractive index and dielectric constant and the thickness of the layer. The following already applies to the total reflection factor for both the p- and the s-polarized portion of the values, from US Pat. No. 4,052,666 well-known formula:

wherein for r ₁₂ the reflection factor between the medium located above surface 1 and the medium of layer 2 and for r ₂₃ the reflection factor between the medium of layer 2 and medium 3 of the corresponding polarization direction are used. The reflection factor r ₁₂ can be determined in a known manner using the Fresnel formula. The size known from literature

describes the phase shift when passing through the layer once, where λ _{o is} the free wavelength, d is the thickness of the layer and ε _{r1 is} the complex dielectric constant for the medium above surface 1, in the case of air a value of 1 as a good approximation for this value applies, and ε _{r2 is} the complex dielectric constant for the medium of

Layer 2 mean.

By means of the evaluation method described below, the real and the imaginary part of the complex quantities r ₂₃ for the two polarization directions p and s can be determined and thus the quantities sought, such as the complex refractive index or dielectric constant and the thickness of the layer 2, can be determined.

The equation for the calculation of the total reflection factor R applies to all layer thicknesses, i.e. also for the thickness d = 0, which gives the following relationship:

The complex reflection factor r ₂₃ can be surprisingly determine from the complex reflection factor r ₁₃ between the medium above the surface 1 and the medium 3, again for both polarization directions.

Thus, only the reflection factors between the medium 1 and the medium 3 need to be known for the solution of the entire problem.

Three conditions for these four unknown quantities, the real and imaginary part of the reflection factor r _13, both for the p- and for the s-polarized part of the signal or the electromagnetic wave, can be measured by measuring the uncovered surface once (layer thickness d = 0) determine by means of the device according to the invention.

If, for example, antenna 5 is used for the p-polarized wave and antenna 6 for the s-polarized wave, the following applies when d = 0:

Ir ₁₃ , p: = A _o

Ir ₁₃ , s; = B _o arg (r _13, p) - arg (r ₁₃ , s) = φ or arg (r ₁₃ , p) = arg (r ₁₃ , s) + φ.

This leaves one size, for example arg (r _{13, s} ) undetermined.

A further measurement is necessary to solve the entire system of equations and thus to be able to determine the quantities sought, such as complex refractive index and dielectric constant and thickness of the layer. This takes place in the evaluation by means of a further measurement to be carried out only once, in which the surface is covered by a layer, of which at least one of the sizes, such as the real or imaginary part of the refractive index or dielectric constant or the layer thickness, is known. Three sizes are measured again, with a maximum of two further unknowns, so that the entire system of equations can be solved. In the specific case of the measurement of a layer covering a traffic area, no artificial application of a layer and thus a blocking of the traffic area for the traffic is necessary. It is sufficient to carry out the measurement during a rain, since the complex refractive index or dielectric constant of water is known, with the only requirement being that no salt is scattered. Due to the sensitivity of the system of equations this measurement can be carried out with a small layer thickness d <<λ / 4.

If the above-mentioned further measurement is carried out with a layer of which the complex refractive index or dielectric constant is known, as in the case of water on a roadway, only two variables need to be measured and taken into account in the calculation.

In reflectometry, it is common to use the quotient of the total reflection factor for the p-polarization R _p and for the s-polarization R _s for the evaluations. This results in the following system of equations for the case described here, namely that arg (r _12.5 ) and the layer thickness d are unknown:

A _o

= f (arg (r ₁₃ , s), d) = - cos φ

B _o

A _o

= g (arg (r ₁₃ , s), d) = - sin φ

B _o

The measured quantities are A _o , B _o and φ; f and g symbolize that the real and imaginary part of the quotient of the total reflection factors depend only on arg (r _{13, 5} ) and d. This system of equations is so complicated that it is only numerically solved for arg ( _, ) and d

can.

The advantages of the device according to the invention, including the calibration and evaluation method, compared to a known receiver with heterodyne reception and one or more local oscillators connected therewith, lie in the fact that the device according to the invention requires significantly less circuitry. Since the two input signals, which come from the same source, are mixed together, this circuit is also largely insensitive to small fluctuations in the frequency of the input signals. If the transmitter is amplitude modulated, all advantages of the known synchronous detection can be exploited. For the calibration it is only necessary to align the antennas once and to carry out the measurements described above. The determination of the reflection behavior of the surface can in the concrete case of the determination of layers on traffic areas in the current loading drive, which minimizes disturbances in the flow of traffic and avoids the blocking of traffic areas.

Claims:

Claims

PATENT CLAIMS

1. A device for determining the amplitude and phase size of two differently polarized signals picked up by two separate receiving antennas, in particular signals in the microwave range, of a reflectometric measuring device, characterized in that the receiving antennas (5, 6) with the inputs of a power divider (9, 10, 11, 12) and power combiner (13, 14) are connected, the four outputs of which are connected to detectors (20, 21, 22, 23), two of which are connected directly to the signals with an associated receiving antenna (5, 6), and the other two are acted upon by a mixture of these signals, one of these latter detectors (21, 22) being acted upon by a mixture of these signals, in which one of the two signals is rotated in phase.

2. Device according to claim 1, characterized in that the input side with the receiving antennas (5, 6) connected power dividers (9, 10) are connected with their one outputs each with a detector (20, 23) and the second outputs of these power dividers (9, 10) are connected to the inputs of further power dividers (11, 12), one of whose outputs with an input of an associated power combiner (13, 14) and each of the second outputs of the power dividers (11, 12) with that power combiner (13 , 14), the other input of which is connected to the other power divider (11, 12), a phase shifter (15) being arranged in at least one of these lines and the outputs of the power combiners (13, 14) having detectors (21, 22 ) are connected.

3. Device according to claim 1 or 2, characterized in that the detectors (20, 21, 22, 23), which are preferably formed by diodes, are preceded by attenuators (16, 17, 18, 19).

4. Device according to one of claims 1 to 3, characterized in that between the power divider (12) and the power combiner (13) switched phase shifter (15) causes a phase shift of -90 °.

5. A method for calibrating a device according to one of claims 1 to 4, characterized in that in a first step the receiving antennas and the transmitting antenna (s) are aligned directly with one another and the amplitude values A _o and B _o according to the following relationship

are calculated, where D is the distance between the antennas and H is the normal distance of the receiving antennas from the surface, the reflection of which is to be examined, after which reflection measurements, each with a reflection-free input of the power divider connected to the antennas in the operating state, and a measurement in which one of the two antennas is connected to each of the two inputs.

6. The method according to claim 5, characterized in that the antennas (4), (5) and (6) are brought into their final orientation and the amounts of both the p- and the s-polarized portion of the wave s- and p-polarized parts of the wave and the phase difference between the s- and p-polarized parts of the waves reflected from the uncovered surface and then the amounts of the s- and p-polarized parts of the wave and the phase difference between the s- and p-polarized proportions of the waves reflected by a surface covered with a layer with a known refractive index or dielectric constant, preferably rainwater, are measured, the measured values and phase of the reflection factors of the uncovered surface, eg a dry roadway.

7. Measuring method carried out with a device calibrated according to claims 5 to 6 according to claims 1 to 4, characterized in that the amounts of the s- and p-polar parts of the wave and the phase difference between the s- and p-polarized parts measured with reference to the surface covered with a layer and from these measured values taking into account the complex reflection factors of the uncovered surface, both the real and imaginary part of the refractive index or dielectric constant as the thickness of the layer can also be determined.